Avoids a potentially shortening size_t to int cast in strauss_wnaf_
Avoids a potentially shortening size_t to int cast in strauss_wnaf_

Summary: Backport of secp256k1 [[https://github.com/bitcoin-core/secp256k1/pull/841 | PR841]].

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8143

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